FIGS. 1A and 1B are illustrations of a network structure containing a switch stacking. The stacking of switches (layer 2 switches) is to display a plurality of switches as a single switch by stack-connecting the plurality of switches. The stacking of switches may be referred to as virtualization of switches. FIG. 1A illustrates a physical structure of a network. In FIG. 1A, four switches (SW-A through SW-D) are connected through stack links L1 through L4. FIG. 1B illustrates a logical structure of the network of FIG. 1A. In FIG. 1B, the four switches that are stack-connected are displayed logically as a single switch V. In this case, a user can manage the four switches (SW-A through SW-D) integrally as a single switch (the switch V).
In the switch stacking, generally a redundant path is provided as a stack link. This is to permit a frame relay by using the redundant path as an alternative path when a malfunction occurs in one of the stack links. In FIG. 1A, the stack link L4 corresponds to the redundant path. For example, if the stack link L3, which connects between the switch SW-C and the switch SW-D, is out of use due to a failure, frames are sent to the stack link L4 to detour the stack link L3, thereby permitting communications between the switch SW-C and the switch SW-D.
However, if a loop is formed by the redundant path in the network, there is a problem in that a frame storm may occur if an appropriate action is not taken.
As an appropriate action for eliminating such a problem caused by a loop formed by a redundant path, there are the following conventional methods.
1. A control method according to STP (Scanning Tree Protocol).
2. A control method by including TTL (Time to live) in a frame.
3. A method of deciding a relay path at a frame transferer.
FIG. 2 is an illustration for explaining the control method according to STP. According to STP, a redundant path is appropriately cut (blocked) so that a network loop is not formed. In FIG. 2, a port connected to the stack link L4 is blocked in each of the switches SW-A and SW-D. According to STP, even if a problem occurs in an unblocked link and a communication between some switches cannot be performed, the communication between the switches concerned is permitted by switching the path by enabling the redundant path, which has been blocked.
FIG. 3 is an illustration for explaining the control method by including TTL in a frame. According to this method, TTL is included in a frame and the TTL is subjected to subtraction each time the frame passes one of switches so that the frame is discarded when the TTL becomes equal to zero. As a result, the frame is prevented from circulating in a network infinitely, which substantially eliminates formation of a loop. In the example of FIG. 3, a frame F1 containing TTL, which is equal to 1, is sent from the switch SW-D in a direction toward the switch SW-D. The frame F1 is not sent to a stack link beyond the switch SW-A, and a frame F2 is not sent to a stack link beyond the switch SW-B.
FIG. 4 is an illustration for explaining a method of deciding a path at a frame transferer. According to this method, a relay path within a stack is decided by one of switches contained in the stack, which one receives the frame first. In FIG. 4, a path is decided by the switch SW-D, and a frame F3 containing information (path information) regarding the decided path is sent to the switch SW-A. The switch SW-A performs a relay process in accordance with the path information contained in the frame F3.
Moreover, other methods are suggested as a technique to solve the loop problem, such as, for example, in Japanese Laid-Open Patent Application No. 2008-177677.
However, in STP, there is a problem in that a link in a blocked state cannot be utilized. That is, because frames other than a control frame are not transferred by the link in a blocked state in a normal condition, an average hop count becomes large. As a result, a load to a link shared by switches is increased, which generates frame congestion.
Moreover, there is a problem in that the method to include TTL in the frame is effective only for a switch corresponding to the TTL concerned. That is, TTL is not standardized in a layer 2 frame.
Further, in a case where a path is decided at a frame transferer, each switch is required to process a special format frame containing relay information. A function to register a path and seek for a path is required for each switch.